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Previous HitGASNext Hit HYDRATE DISTRIBUTION AND DYNAMICS ON HYDRATE RIDGE, CASCADIA ACCRETIONARY COMPLEX

A.M. Trehu and the Leg 204 Science Party
 College of Oceanic and Atmospheric Science, Oregon State University

Previous HitGasNext Hit Previous HithydratesNext Hit are widespread in marine sediments on continental margins at water depths greater than 300-500 meters. Accurate quantitative estimates of how much hydrate is present in the sediments, however, are rare and difficult to obtain because much of the Previous HitgasNext Hit hydrate has dissociated by the time samples have been retrieved. ODP Leg 204 was only the second ODP leg dedicated to quantifying the abundance of Previous HitgasNext Hit Previous HithydratesNext Hit and understanding the dynamics of their formation in marine sediments (the first was Leg 164 to the Blake Ridge offshore North Carolina in 1994) and provided an opportunity to test several new techniques for sampling, preserving and quantifying Previous HitgasNext Hit Previous HithydratesNext Hit. Nine sites were drilled and cored on or near southern Hydrate Ridge, a topographic high in the accretionary complex of the Cascadia subduction zone, located approximately 80 km west of Newport, OR. All sites fell within a previously-acquired, high-resolution 3D seismic survey, which imaged the “plumbing” of this Previous HitgasNext Hit hydrate system and served as a road map to guide site selection. By integrating results from several techniques with different spatial scales and sensitivity to in situ Previous HitgasNext Hit hydrate content, we were able to obtain the first well-constrained quantitative estimates of how Previous HitgasNext Hit hydrate and free Previous HitgasNext Hit are distributed throughout this system.

Previous HitGasNext Hit hydrate distribution is very heterogeneous. Massive Previous HitgasNext Hit hydrate, comprising ~25% of the total volume, is restricted to the upper 20-30 meters beneath the seafloor at the summit, where bubbling methane vents and authigenic carbonates had previously been observed. Coexistence of Previous HitgasNext Hit hydrate, free Previous HitgasNext Hit and highly saline pore waters in this region requires that Previous HitgasNext Hit hydrate formation here be rapid; seismic and geochemical data indicate that an anomalously permeable stratigraphic horizon channels Previous HitgasNext Hit-rich fluids from deeper within the accretionary complex to form this focused deposit. This deposit is probably typical of similar shallow, concentrated Previous HitgasNext Hit hydrate deposits in which Previous HitgasNext Hit is focused from a within a large volume through a stratigraphically or structurally controlled conduit, as is observed in the Gulf of Mexico and at other sites characterized by deep hydrocarbon reservoirs and fluid migration.

Elsewhere, Previous HitgasNext Hit hydrate occurs primarily in patchy clusters of veins and lenses up to a few cm thick. Although the regional average Previous HitgasNext Hit hydrate content of the sediments is lower than previously thought (~2%), relatively large amounts of Previous HitgasNext Hit hydrate (up to 20% of pore space over thicknesses of ~ 10 m) occur near the base of the Previous HitgasNext Hit hydrate stability field and within coarse-grained layers. This type of Previous HitgasNext Hit hydrate distribution is typical of Previous HitgasNext Hit hydrate formed primiarly from locally produced methane.

Bibliography:

Trehu, A.M., G. Bohrmann, F.R. Rack, M.E. Torres, Proc. ODP, Init. Repts. 204, [CD-ROM]. Available from: Ocean Drilling Program, Texas A&M Un., College Station TX 77845-9547, USA, 2003.

Trehu, A.M., G. Bohrmann, F.R. Rack, M.E. Torres, and Leg 204 Science Party, Previous HitGasNext Hit hydrate distribution and dynamics beneath southern Hydrate Rige: ODP Leg 204, JOIDES Journal, v. 29, Fall, 2003.

Trehu, A.M., Bohrman, G., Rack, F.R., Collett, T.S., D.S. Goldberg, P.E. Long, A.V. Milkov, M. Riedel, P. Schultheiss, M.E. Tores, N.L. Bangs, S.R. Barr, W.S. Borowski, G.E. Claypool, M.E. Delwiche, G.R. Dickens, E. Gracia, G. Guerin, M. Holland, J.E. Johnson, Y-J. Lee, C-S. Liu, X. SU, B. Teichert, H. Tomaru, M. Vanneste, M. Watanabe, J.L. Weinberger, 2004. Three-dimensional distribution of Previous HitgasNext Hit hydate beneath southern Hydrate Ridge: constraints from ODP Leg 204, Earth and Plan. Sci. Let, in press.

Torres, M.E., K. Wallmann, A.M. Trehu, G. Bohrmann, W.S. Borowski, H. Tomaru, Previous HitGasNext Hit hydrate dynamics at the Hydrate Ridge southern summit based on dissolved chloride data, submitted to Earth and Plan. Sci. Let., Jan. 2004.

Trehu, A.M., P. Flemings, N. Bangs, M. Torres, J. Johnson, M. Riedel, C-S Liu, Lithostatic Previous HitgasTop pressures and venting at southern Hydrate Ridge, submitted to Geology, April, 2004.